THERE will be two full moons this month; one on the 1st and the other on the 31st. This second one is referred to nowadays as a ‘Blue Moon’. The Moon takes around 29.5 days to orbit the Earth once and apart from February all months have either 30 or 31 days, meaning it is possible to have two full moons in a calendar month.

This use of ‘blue moon’ only has a mythology dating back to the 1940s when a letter, sent to the American astronomy magazine ‘Sky and Telescope’, asked the question ‘what is a blue moon?’.

The answer was that it is the second full moon in a calendar month. The publishers, realising they had given the wrong answer, retracted their original statement. However, it was too late; the genie was out of the bottle, and for the last 75 years everyone has accepted this definition.

I have my own theory about the term ‘blue moon’. Each month I give the name of the next full moon, and as I often say, these names go back to the times of the monastic period in Britain around 1,000 years ago.

The monks, who were amongst the cleverest people around during the middle ages due to their vast monastic libraries, knew full well that in some years there were 13 full moons rather the normal 12.

Monks liked order in their lives. They kept diaries and special dates were always marked in red ink; you may have have heard of the phrase ‘a red letter day’ which is a monastic saying going back over 1,000 years. I just wonder if, when this 13th full moon occurred, it was marked in diaries in blue ink as a source or irritation because it messed up a very orderly system.

I once checked the reference library in York minster without success, but maybe someone in the future will check the Vatican library and discover a monk’s diary with a full moon marked in blue ink!

During October evenings the autumn constellations are at their best. The Plough is to all intents and purposes at its lowest in the north, although it is still reasonably high in the sky. Cassiopeia is of course now very high and not far from the overhead point. The two bright stars, Vega, which was overhead in summer and is part of the summer triangle, and Capella, which will be overhead in winter, are roughly the same height in the sky. Vega is in the north west while Capella is in the north east.

Ophiuchus is now becoming low in the west and is replaced by another faint group Cetus (the Whale or Sea Monster). Perseus and Taurus, two splendid looking winter groups, are becoming far more conspicuous. The summer triangle of Altair, Deneb and Vega is still easy to see, as is the Milky Way which is directly overhead. The southern skies are now dominated by Pegasus.

On maps, the square of Pegasus looks easy to find, but because the four stars are not the brightest in the sky they don’t stand out. However, once recognised it will be easy to find again. One way of finding Pegasus is to look below the ‘W’ of Cassiopeia.

The brightest star in the square is the top left hand star, Alpheratz. For some unknown reason this star was transferred to the neighbouring constellation of Andromeda. Alpheratz is nearly the same brightness as the North Star, while the other three stars of the square are a little bit fainter.

In mythology Pegasus was the flying horse ridden by the hero Perseus who, after killing the Medusa, returned to rescue the princess Andromeda who was chained to a rock waiting to be eaten by the Kraken. Perseus still had the head of the Medusa which he pointed at the monster, turning it to stone. All these characters can be found in the sky, including the sea monster.

Andromeda’s main stars are arranged in a somewhat irregular line, running from the Square of Pegasus towards Perseus. However, the main interest in Andromeda is the Andromeda Galaxy, which is visible to the naked eye if you are away from city or town lights.

To find the galaxy, locate the star Beta Andromeda and follow the line of stars; Alpha (or Alpheratz) is the top left hand star in the square of Pegasus, next comes Delta then Beta Andromeda. Looking upwards you will see two fainter stars Mu and Nu, and the Andromeda Galaxy is just to the right hand side of Nu.

The Andromeda Galaxy is the most distant you can see with the naked eye. It is about 2.5 million light years away.

Below the line of stars that forms Andromeda is a fairly bright star called Hamal, which is the brightest star in the constellation of Aries (the Ram). There should be no problem finding Hamal as it is quite isolated. In mythology, Aries represents the ram whose golden fleece was sought by Jason and the Argonauts.

The main claim to fame for Aries is that it used to be the first sign of the Zodiac. Around 2,000 years ago the point where the Sun passes the celestial equator from south to north in the sky was in Aries. This point is called the Vernal equinox. However, because the Earth wobbles very slightly over a very long period of time, this point in space has moved into the neighbouring constellation of Pisces (the Fishes). Astronomers still call this place in Pisces ‘The First Point of Aries’.

Between Aries and Andromeda is a small but quite noticeable group of three stars. This is Triangulum (the Triangle), and it is one of the few constellations whose name describes what it actually looks like.

The area below the square of Pegasus is rather barren, with faint constellations such as Aquarius, Capricornus and Pisces, but there is one bright star, Fomalhaut in the constellation of Piscis Austrinus (the Southern Fish). Use the two right hand stars of the square and draw a line down towards the horizon and the brightest star you will see is Fomalhaut. Otherwise the group is unremarkable.

Another of the large but faint groups in the autumn is Cetus the Whale. In mythology, Cetus was identified with the sea monster sent to eat the princess Andromeda, which was turned to stone when Perseus arrived in the nick of time to rescue her. More recently it has become a harmless whale.

It is a difficult area to identify but there is one star in Cetus that is worth looking out for. It is the normally faint star omicron Cetus. The star is better known by its proper name of ‘Mira the Wonderful Star’. The star varies in brightness every 331 days; sometimes it becomes so bright that it totally transforms the area. Mira is a red giant star and when it does become bright it is very noticeably red in colour. It will remain at its brightest for many weeks. Sometimes when at its faintest it is hard to see, and a telescope will be needed to find it.

The Planets in October:

Jupiter and Saturn continue their journey lower in the south west sky. Jupiter is the bright white dot with Saturn above and to the right as a bright yellowish looking dot. By the end of the month both planets will only be visible for about an hour before they set. While the gas giants get lower in the sky the southern sky is graced by a bright red dot, Mars, the red planet, the god of war. Mars is looking very red at the moment and is difficult to miss. Just after midnight the bright white dot which is Venus can be seen rising in the south east. Apart from the Sun and Moon, Venus is the brightest object in the sky. The only other naked eye planet, Mercury, is still too close to the Sun to be seen.

Meteor Showers:

There are two meteor showers this month. The name of any meteor shower is taken from the constellation the meteors appear to come from. The Draconids, which radiate from the constellation of Draco the Dragon, peak on the night of the 7/ 8 and are associated with comet Giacobini-Zinner. The Draconids normally produce about 15 meteors per hour but some years they have significant outbursts so they are always worth watching. Harry Potter fans might recognise the name ‘Draco’ as one of the villains in the books. The other meteor shower this month is the Orionids, which radiate from Orion and are associated with the famous comet Halley. The peak will be on the night of October 20th / 21st when around 20 meteors per hour might be seen.

Phases of the Moon for October:

Full Moon 1 and 31; Last Quarter 10; New Moon 16; First Quarter 23.

For astronomers everywhere Sunday 25 October is a great day, as British Summer Time ends and there is an extra hour of night time to observe the stars.

Due to the current Coronavirus situation there will be no meetings of the Earby Astronomical Society until further notice.